This invention relates to a method of depositing multilayers of semiconducting organic polymers.
Gas sensors based on the use of chemically sensitive semiconducting organic polymers are well known. GB-2,203,553-B describes a class of gas sensor which consists of two electrodes and a layer of semi-conducting organic polymer deposited on and between the electrodes such as to effect a semiconducting electrical connection. The electrical properties of the semiconducting organic polymer are affected by the presence of a gas or volatile species, and therefore the presence of a gaseous species may be detected by monitoring the change in an electrical property on exposure of the gas sensor to the gas. GB-2,203,553-B discloses the application of an AC electric signal across the gas sensor and the detection of various impedance characteristics, such as conductance. Maisik et al (Maisik, JJ, Hooper, A and Tofield, BC) JCS Faraday Trans. 1, 1986, 82, 1117-26 discloses a gas sensor wherein a DC electric signal is applied, and the DC resistance is detected.
In previous reports of semiconducting organic polymer based gas sensors the polymer has been deposited by electrochemical polymerisation of a solution containing the monomer and a counter-ion.
A major problem with gas sensors which employ a single semiconducting organic polymer to bridge a pair of electrodes is a lack of sensitivity. In general, a large gap between the electrodes is desirable ("large" in this context being greater than ca. 100 um) since experimental evidence shows that a large surface area of polymer can result in enhanced sensitivity. A large surface area of polymer also results in a range of resistances that are easily measurable. However, in practice, the maximum spacing between the electrodes (determined essentially by the length of the polymeric chains) is typically between 1 and 5 um. Wider spacings may be achieved, but at the expense of sensor performance, since, as the electrode spacing increases much beyond the length of the polymeric chains, various mechanical properties worsen and the resistance of the polymer increases dramatically--sometimes prohibitively so. The poor mechanical properties are due to a "necking" effect in the polymer deposit. This effect is illustrated in FIGS. 1 and 2 by reference to a gas sensor with a large separation between the electrodes 10a and 10b. The electrodes are supported by a substrate 12, and the semiconducting organic polymer 14 has been deposited electrochemically in an uneven manner, with "necking" apparent, both in a plan and a cross-sectional view. Gas sensors of this type are often very brittle and production of the gas sensor suffers from poor reproducibility. Thus International Application No. 93/03355 limits the scope of the multiple gas sensor device claimed to one in which the electrodes are spaced up to 25 um apart, and U.S. Pat. No. 4,721,601 discloses a microelectronic device comprising at least two electrodes separated by less than 2 um.
International Application No. 86/01599 discloses the use of more than one semiconducting organic polymer in a single gas sensor, wherein polypyrrole is employed as a substrate upon which another semiconducting organic polymer is deposited as a coating. Both of these semiconducting organic polymers are polymerised electrochemically. Using such a method, a gas sensor was produced which employed two platinum wire electrodes spaced 200 um apart. However, the electrochemical deposition of the polypyrrole substrate is now found to be not an optimal method, since irrespective of the design of the sensor, the polypyrrole tends to be polymerised as a globule. As a result, the sensor exhibits poor mechanical properties, such as poor adhesion between the polypyrrole and the electrodes, and between the polypyrrole and the second polymer coating. A lack of mechanical robustness is a considerable drawback, since a practical gas sensor should be able to withstand day-to-day usage, including changes in temperature and humidity. Additionally, the adsorption of gaseous species onto the polymer surface (the very process upon which gas detection depends) induces substantial conformational changes in the polymer which can lead to internal stressing.